Using neutron diffraction, movement of charged atoms arranged as “stripes” was captured for the first time.

Data points indicating the strength of long-range charge order (top) show that the static stripe order disappears at 240 K, while the data related to short-range dynamic correlations (bottom) show a gradual decay at much higher temperatures. Inset pictures of charge stripes depict schematically the type of order that exists above and below the transition temperature.

The Science

Planes of charged atoms in metal oxides can appear as stripes across the background of the full atomic array. A new experimental technique revealed disordered versions of the stripes that persist well above the temperature where the rigid version of the stripes melts away, allowing correlation of electronic properties with atomic structure.

The Impact

To understand the quantum effects that arrange electrons in solid oxides, giving them unique properties such as magnetism and superconductivity, requires careful measurement of the electron structure and atomic positions. This research shows that neutron and x-ray scattering can be used to track subtle dynamic structures across phase transitions with powder samples that are easier to prepare and measure than large single crystals.

Summary

Metal oxide crystals involving elements in the middle of the periodic table often exhibit charge arrangements with periodicities larger than that of the atomic structure. When drawn from the side in a two dimensional view, the planes of charged atoms appear as stripes across the background of the full atomic structure. These stripes are related to the charge ordering of the valence electrons in the atoms and play an important role in novel properties that are useful in electronic devices. Above the transition temperature where static charge ordering disappears, local stripe pieces may exist, modeled during characterization of the material as wavy stripe patterns that are moving. Measuring such features usually requires large single crystals and difficult energy loss scattering techniques, and even these techniques can fail because of low signal strengths. This research shows that there is a structural signature from the bond lengths of the coupled charged atoms even as their positions fluctuate. Sorting the bond lengths by bond type and by the specific way these vibrate, and then tracking these as a function of temperature, is a key element of the new technique. The widths and intensities of certain peaks in the powder diffraction pair distribution function, associated with charge order, have been shown to correlate with anomalies in optical conductivity (indicative of exotic non-metallic behavior), while other unrelated peaks show temperature dependent behavior typical of non-correlated vibrating atoms.